Using XPath

Hi,

I'm trying to evaluate a XPath against a XML-Document. Now i fount the DOM
Level 3 implementation documentation.

http://developer.mozilla.org/en/docs/Introduction_to_using_XPath_in_JavaScript

and I simply want to know from which version on is this supportet, Gecko or
Firefox. Because I would like to write Code that runs in older Versions
also.

thx
Spanky